In recent years, refractory shotcreting has become a major method of installation for lining all types of refractory surfaces and for repair and maintenance of linings in steel, non-ferrous metal, chemical, mineral and ceramic processing plants.
Shotcreting is usually classified according to the process used, i.e., wet-mix or dry-mix spraying. A wet-mix refractory shotcrete process generally consists of thoroughly mixing a refractory material and water to produce a pumpable mixture. The mixture is then introduced into a delivery hose, and moved by pumping or conveying by compressed air to a dispensing (i.e., spraying) nozzle. A dry-mix refractory shotcrete process generally consists of mixing and conveying through the delivery hose the dry constituents of the refractory, and then introducing water under pressure at the nozzle prior to application of the refractory to the target surface.
In both processes, an admixture is typically added to the refractory castable at the nozzle. The admixture is a material that is used as an ingredient to initiate and accelerate the thickening and setting of the refractory castable so that it will not slump or sag when applied to vertical or overhead surfaces. Specifically, the admixture is a set-controlling material, commonly referred to as an "accelerator," that decreases the time required for the refractory to stiffen. In this respect, accelerators are generally used to speed tip the rate of stiffening of the refractory castable.
Sodium silicate is a conventionally known accelerator used in shotcreting processes. Sodium silicate provides very rapid acceleration of the setting of most cementitious materials and finds advantageous application in many types of refractory castables. However, in some refractory applications, and with some refractory castables, sodium silicate provides less than desirable results when used as an accelerant in a shotcreting process. In one respect, the rapid setting does not allow for surface shaping or contouring of the refractory, in that it basically hardens on contact. In another respect, the rapid setting produced by sodium silicate in some applications (i.e., with some materials or for some uses) is actually too rapid and results in a layered or stratified refractory lining. As the sodium silicate accelerated cementitious material is sprayed onto a surface, it basically sets when it hits the surface producing a thin layer of refractory. Subsequent passes (spraying) over this area puts another layer over the first layer, building up the refractory in layers with each subsequent pass. Basically, each layer is only loosely bonded to adjacent layers, thereby producing a material having poor matrix integrity.
In some instances, such a structure is adequate for the desired application. In other situations and applications, however, such a material is less than satisfactory. One situation, for example, is in shotcreting blast furnace trough castables. Refractory blast furnace trough castables must be capable of withstanding high temperatures, resist mechanical erosion and slag corrosion. Trough castables are typically low cement refractory materials composed primarily of a high alumina aggregate (brown fused alumina or bauxite, for example) and silicon carbide. A low cement level is used to minimize the reaction with blast furnace slags. In this respect, slag reacts with the lime in the cement, thus corroding the refractory lining. The high alumina aggregate and silicon carbide help resist attack from blast furnace slags. The silicon carbide also acts as an antioxidant and a volume stabilizer. Blast furnace trough castables also typically include other materials such as carbon, cement, fine aluminas and/or silicas or metals. The carbon is a non-wetting compound to iron and slag, and increases the thermal conductivity of the castable thus providing better thermal shock resistance to the castable. The fine aluminas and silicas help promote good flow properties, corrosion resistance and hot strength. Metals are added as antioxidants to protect, to aid in dryout and to enhance the strength.
Examples of castable refractory compositions suitable for wet-process shotcreting of blast furnace troughs include North American Refractories Company SHOTKASTTRW.TM.,SHOTKASTTRC-SR.TM.,SHOTKASTTRC-OR.TM., SHOTKAST TRC.TM. and SHOTKAST XZR-IR.TM. refractories. All of these mixtures are based on high alumina aggregate and silicon carbide compositions with varying amounts of carbon, cement, fine aluminas, fine silicas, metals and dispersing agents.
When sodium silicate is used with a low cement blast furnace trough castable, it provides very rapid setting of the refractory castable, as indicated above. In fact, the acceleration is too fast and does not allow sufficient time for layers of the sprayed material to "knit" (i.e., bond) together as it is being applied. This lack of knitting results in a laminated lining and a weakened structure not well suited for the high temperature and corrosive environment of a blast furnace. In other words, sodium silicate accelerated castable systems provide poor matrix integrity. In addition, the quick setting of the low cement refractory castable, when sodium silicate is used, does not allow for any surface scraping or contouring to be done after installation; the material simply hardens too quickly.
It is therefore thus desirable to provide an admixture which will not adversely affect the properties and characteristics of refractory castables, yet will allow shotcreting of refractory castables.
The present invention provides an admixture for use in a shotcreting process of refractory castable which prevents slumping or sagging of the refractory castable, and at the same time, allows "knitting" of sprayed layers of the refractory.